A conventional chipper for converting veneer wastes into useful chips is described by Elmendorf in U.S. Pat. No. 2,570,926. In Elmendorf, multiple layers of scrap wood veneer are transported on a conveyor and discharged onto and across a stationary anvil surface. At the discharge edge of the anvil, the veneer is contacted with a chipping blade that rotates transverse to the conveyor and strikes the veneer adjacent the anvil surface. A problem well recognized in the art in such apparatus when used to cut resilient or springy type materials is that the materials tend to rebound from impact with the blade. The rebound action adversely affects chip quality. Elmendorf thus provides a hold-down mechanism employing a plurality of shoes extending across the width of the conveyor to rest on the veneer mat and hold it onto the anvil surface during cutting.
Hold-down mechanisms such as proposed by Elemdorf generate a number of difficulties. One difficulty relates to the framework necessary to support the hold-down apparatus. This supporting framework typically overlies the conveyor and limits how much veneer may pass under it. Such flow restriction limits the ability of the chipping apparatus to handle the substantial surges in the level of veneer material on the conveyor that often occur. Such flow limitations result in spillage and loss of veneer as well as jamming of the machinery.
Another difficulty with conventional hold-down apparatus is that the closeness that the hold-down shoes can approach the cutting blade is limited because normally the hold-down apparatus is pivoted from a relatively long supporting arm, thereby causing the tip of the hold-down mechanism to trace an arcuate path in response to flow variation of material across the anvil surface. The tip of the hold-down mechanism closest to the blade must be fixed a sufficient distance away from the blade to accommodate the horizontal component of the arc that the hold-down tip will travel under varying flow conditions. With respect to chipping wood veneers, this adjustment away from the chipping blade has a significant negative impact on chip quality. A substantial percentage of pin chips and splinters are produced that are undesirable in many preferred uses for the chips, such as pulping.
Plough, in U.S. Pat. No. 3,718,169 describes an apparatus for chipping scrap wood veneer that includes a deeply slotted hold-down roll supported upon a framework attached to a feed drum, both bearing upon the veneer as it travels along the feed conveyor. Plough includes a plurality of substantially parallel fingers mounted on a cross-shaft supported upon the chipper frame. The fingers extend through the slots on the crush roll to hold veneer onto the anvil surface near the knife blade. The apparatus includes a flexible air bag for exerting a downward force on the fingers and, hence, on the veneer fed to the chipper. Again, a disadvantage of this arrangement is that the physical supporting structure limits the capacity of the apparatus to handle surge flows of veneers. The finger arrangement also is such that the fingertips travel an arcuate path as they pivot to accommodate veneer flow. Thus the hold-down tips must be spaced back from the blade, reducing their effectiveness, since veneer rebound is not completely controlled.